Determination of true stress-strain data behond the necking point and fracture loci of vintage steel pipes, using digital image correlation and finite element analysis

制造业、交通运输业、发电业和许多其他行业都严重依赖石油和天然气产品的高供应。管道是运输和输送这些重要产品的主要手段。然而，像任何结构系统一样，它们很容易受到生命的破坏和退化。根据许多因素，管道行业已经定义和标准化了各种各样的评估和检查技术，以考虑其系统的完整性。通常，特征（如凹痕、裂纹、皱折和屈曲）和金属损失（如腐蚀）的重要性决定了维护周期的严重性，以确保系统的安全完整性。近年来，人们对老式（1970年以前生产的）钢管材料特别感兴趣，以研究它们在不同载荷条件下（如单轴拉伸、压缩和弯曲）对大变形的微观和宏观结构行为。考虑管道系统中的大变形/位移可能导致管道材料产生大应变。通常，非常大的应变（超过单轴颈点）存在于皱褶或局部屈曲等特征的区域。这些高水平的诱导应变会导致管道内的损伤积累，危及管道系统的完整性。为了获得更好的管道变形预测，为有限元模型提供更可靠的材料性能是非常重要的。本工作的主要目的是进行单轴拉伸测试，使用数字图像相关（DIC）技术，获得Lankford系数，大范围应变（颈点以外）的应力-应变曲线，以及不同应力-三轴（断裂轨迹）的断裂应变。为了考虑材料的各向异性，将在轴向和周向两种不同的管道方向上对各种古钢管材料进行试验。试样尺寸对所得力学行为的影响也将被研究。

Manufacturing, transportation, power generation and many other industries strongly rely on a high supply of oil and gas products. Pipelines are the main means of transportation and delivery of these vital products. However, like any structural system, they are vulnerable to through life damage and degradation. Depending on many factors, pipeline industry has defined and standardized a wide variety of assessment and inspection techniques to consider the integrity of their systems. Generally, the significance of features (e.g. dents, cracks, wrinkle and buckles) and metal loss (e.g. corrosion) determine the severity of the maintenance period to assure a safe integrity for the system. In recent years, particular interest was aimed at vintage (manufactured before 1970) steel pipe materials to investigate their micro- and macro-structural behaviors to large deformations, under different loading conditions (such as uniaxial tension, compression and bending). The consideration of large deformations/displacements in a pipeline system could result in the generation of large strains in the pipe material. Normally, very large strains (beyond the uniaxial necking point) exist in regions with features such as wrinkle or local buckling. These high levels of induced strains can lead to damage accumulation in the pipe and endanger the integrity of the pipeline system. In order to obtain better pipeline deformation predictions, feeding more reliable material properties to an FE model is of great importance. The main purpose of this work is to conduct uniaxial tensile testing, using a Digital Image Correlation (DIC) technique, to obtain Lankford coefficients, stress-strain curves over a large range of strains (beyond the necking point), and the fracture strains for different stress-triaxialities (fracture locus). In order to consider the material anisotropy, tests will be conducted for a variety of vintage steel pipe materials at two different pipe orientations, axial and circumferential. The effect of sample size on the resulting mechanical behavior will also be investigated.